2015 Littelfuse LED Lighting Design Guide www.littelfuse.com LED Lighting Design Guide While fuses, MOVs, and TVS diodes are critical devices that protect LEDs against these threats, PLEDs perform a different kind of function. They are used in series wired strings of LEDs where a single LED failure (open-circuit) would cause an entire LED string to go dark. Such events can't be tolerated in high reliability applications, and can be very costly in terms of product returns, warranty expense, and ruined customer relations. In a typical application LED's are connected in series and driven by a constant current source as shown in Figure 15 . The constant current source has a maximum (open circuit) output voltage called the "compliance voltage," which appears across an LED th at has an open failure. This will trigger the PLED device to turn on and conduct current around the open LED. The remaining LEDs stay illuminated, and in many applications, the failure of a single LED is not critical or even noticeable. The PLED power dissipation is lower than the LED, thus lowering total system power. Additionally, PLED devices can also protect against reverse polarity ( Figure 16 ). If the power supply voltage is reversed, the PLED5 for example will switch on and bypass the LEDs with only a diode drop needed to turn it on. The reverse current passes through the PLED devices and prevents damage to the LEDs. In this case, the LEDs will not turn on while bypassed, but once normal power polarity is restored, the PLED devices turn off and the LED string will function normally. Control Circuit Control Circuit Control Circuit MT2 MT1 Control Circuit Control Circuit Control Circuit MT2 MT1 Control Circuit MT2 MT1 PLED5 ACTIVATED Constant Current Source PLED5 ACTIVATED Figure 16. PLEDs also protect LED strings from reverse polarity situations. Design Considerations for PLED Protectors (continued) 27